Organic electronic devices provide many potential advantages including inexpensive, low temperature, large scale fabrication on a variety of substrates including glass and plastic. Examples of such devices are organic light emitting devices (OLEDs) and light emitting electrochemical cells (LECs).
Organic light emitting diode (OLED) displays, for example, provide additional advantages as compared with other display technologies—in particular they are bright, colourful, fast-switching and provide a wide viewing angle. OLED devices (which here include organometallic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multi-coloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, WO95/06400, WO99/48160 and U.S. Pat. No. 4,539,570, as well as to “Organic Light Emitting Materials and Devices” edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10: 1-57444-574X, which describes a number of materials and devices, both small molecule and polymer.
In its most basic form an OLED comprises a light emitting layer which is positioned in between an anode and a cathode. In operation holes are injected through the anode into the light emitting layer and electrons are injected into the light emitting layer through the cathode. The holes and electrons combine in the light emitting layer to form an exciton which then undergoes radiative decay to provide light.
In its basic form a light emitting electrochemical cell (LEC) also comprises a light emitting layer which is positioned in between an anode and a cathode. In a LEC, however, the light emitting layer also comprises an electrolyte and a salt(s). Light is generated when holes and electrons, injected from the anode and cathode respectively, combine in the light emitting layer. For general background information on LECs, reference may be made to U.S. Pat. No. 5,682,043 and WO2011/032010.
Screen printing, gravure printing, flexographic printing, dispense printing, nozzle printing and ink-jet printing are cost-effective methods for depositing some, and more preferably, all of the layers of organic light emitting devices. Such techniques are attractive because they are relatively cheap, partly because they can be carried out in ambient conditions and provide high throughput, readily adaptable patterning and the ability to process flexible substrates.
To date, printing is more commonly used to deposit the light emitting layer of organic light emitting devices than the electrodes, and particularly the electrode that is present on top of the light emitting layer. Instead, techniques such as vacuum evaporation of low work function metals have been used for this top electrode, which can greatly increase the complexity and cost of fabricating the device.
The difficultly with printing an electrode on a light emitting layer is that it tends to cause damage to the light emitting layer which in turn decreases the electrical performance of the device in which the layers are present and increases the risk of shorting between the anode and cathode when deposited. The solvents used in inks for printing tend to dissolve components of the light emitting layer which increases the surface roughness of the light emitting layer, damages the light emitting layer/electrode interface and increases the inhomogeneity within the layer, e.g. by dissolving and removing components of the layer. The thermal treatment required to remove the solvent from a printed ink can also cause damage or worsen the damage caused by contact with the solvent.
These effects are exacerbated in light emitting layers comprising polar and non-polar polymers (e.g. PEO and light emitting polymer) as well as small molecule polar components. Such layers are particularly common in LECs. Light emitting layers comprising polar and non-polar polymers tend to have relatively rough surfaces due to the tendency of the polymers to phase separate. If an ink comprising a solvent is then applied to this surface it almost always dissolves the polar polymer and polar components to a greater extent than the less polar polymer and the inhomogeneity is worsened further. Leaching of the polar components also occurs. Using inks comprising non-polar solvents does not, however, solve this problem since these inks will simply dissolve or preferentially dissolve the less polar polymer.
Hence there remains a need for improved methods of making electrodes for organic electronic devices comprising a light emitting layer.